1. Field of the Invention
The present invention relates to components for a steel pickling apparatus, such as heat exchanger components for heating hydrochloric acid pickling solutions, and related systems and methods.
2. Description of Related Art
When steel is exposed to oxygen at high temperatures, such as during or after hot rolling, the oxygen chemically reacts with a hot surface of the steel and forms a compound normally referred to as scale. This scale generally must be removed prior to further processing such as cold rolling, drawing, stamping, etc.
Scale can be removed by mechanical methods. However, mechanical scale removal damages the surface of the steel. As an alternative, removal of scale by chemical processing allows for quick and clean removal of scale without surface damage. This method of scale removal is commonly referred to as pickling, and at first was generally conducted using sulfuric acid.
In the early 1960's a major change in high speed pickling technology took place—the introduction of hydrochloric acid (HCl) pickling. Hydrochloric acid became an attractive alternative to sulfuric acid because it could be regenerated and because it permitted higher pickling speeds. However, hydrochloric acid had its disadvantages. One disadvantage is that hydrochloric acid is much more corrosive than sulfuric acid, creating challenges with materials used to handle the hydrochloric acid.
Temperature and concentration of the pickling bath plays an important role in pickling efficiency. The pickling rate considerably increases with an increase in the pickling bath concentration and temperature. Therefore, to increase pickling speeds, high concentrations of hydrochloric acid are heated to high temperatures, as high as approximately 22 weight percent hydrochloric acid and approximately 210° F. (99° C.).
To maintain the high temperature of the pickling bath, graphite heat exchangers may be used. However, these graphite heat exchangers have less efficiency due to cross-contamination of steam and acid, frequent leakages, clogging of pores, and regular need for maintenance.
Alternatively, direct steam injection in the bath may be used to maintain the high pickling bath temperature. However, this leads to pickling bath dilution, which reduces the acid concentration and, thereby, reduces the pickling efficiency.
Using a heat exchanger including polytetrafluoroethylene (PTFE) tubing is also possible. However, installation of a PTFE heat exchanger is not feasible for all pickling applications.
Other types of heat exchangers may be used to maintain the high acid temperatures. For example, a heat exchanger may include metal tubing that transmits heat between a hydrochloric acid solution contacting one surface of the tubing and a heating fluid such as steam contacting an opposing surface of the tubing. Thereby, heat is conducted from the steam to the hydrochloric acid solution. The conductive portions of the heat exchanger that come into contact with the hydrochloric acid solution must have high resistance to corrosion in acidic and chloride-containing environments in order to attain an adequate service life. Additionally, portions of the heat exchanger that act as a medium for heat transfer should have have high thermal conductivity to permit efficient heating of the hydrochloric acid.
Conventionally, tantalum (Ta) is the primary alloy used in heat exchanger tubing for heating highly-concentrated hydrochloric acid pickling solutions used in pickling of carbon steel. Tantalum is one of the most corrosion resistant materials available. At temperatures under 300° F. (149° C.), tantalum is regarded as being inert to all concentrations of hydrochloric acid. Additionally, tantalum has a thermal conductivity of 32 Btu/hr-ft° F. (57.5 W/m-K), which provides for excellent heat transfer between the heating fluid and the hydrochloric acid solution. However, tantalum is expensive (approximately $300/lb; $0.66/g) and has a high density (16.6 g/cm3). Therefore, the cost for producing a desired thickness of tantalum is very expensive.
The excellent corrosion resistance of tantalum heat exchangers allows heat exchanger tubing to remain in service indefinitely. However, due to the high cost of tantalum, only the parts of the heat exchanger that contact the hydrochloric acid are formed from tantalum. Other parts of the heat exchanger are formed from other materials, such as carbon steel.
A pickling line in a steel mill is a very difficult environment. Most heat exchangers fail not because of failure of the tantalum tubing but instead from corrosion of the steel parts or from lack of maintenance. Although solids in the pickling acid should be screened out, the filters may fill up rapidly and are often bypassed. Thus, these heat exchangers can foul and require cleaning. Accordingly, the heat exchangers typically fail in less than 10 years depending on the level of maintenance and the specific application of the heat exchanger.